perf(workstation): debounce + cache commit file-preview hydration

[gfargo-horizon-agent]

What

Adds a ~120ms debounce and a bounded (hash, path, oldPath)-keyed cache to useCommitFilePreviewHydration, so holding j/k over a commit's file list stops firing an uncached git show subprocess per cursor move and previously-visited previews redisplay instantly with no loading flash.

Why

Plane: OSS-595

Sub-item 2/5 of OSS-338. Its sibling hook, useCommitDetailHydration, already got this treatment in #1533. This closes the remaining gap: useCommitFilePreviewHydration (in useDiffHydration.ts) was still fetching uncached on every selectedDetailFile change.

How

• useCommitFilePreviewHydration: added a useRef-backed, size-capped (100 entries, FIFO eviction) cache keyed by ${hash}\0${path}\0${oldPath ?? ''} (oldPath is included because renamed files pass different paths to git show).
• Debounced the fetch by 120ms; rapid re-selection clears the prior timer via the existing active-flag cleanup, mirroring useCommitDetailHydration.
• On a cache hit: setFilePreview(cached) + setFilePreviewLoading(false), no fetch, and setFilePreviewLoading(true) is never called.
• Preserved the active-flag stale-response guard, the safe() wrapper, the no-selection bail, and the dependency array exactly — no change to the exported hook signature or the app.ts call site (only an internal useRef was added).
• Updated the module and hook doc comments to note this loader is no longer a verbatim lift (the other four diff loaders remain verbatim).
• No changes were needed to useCommitDetailHydration.ts — that half of the ticket was already merged in perf(workstation): debounce + cache commit detail hydration #1533.

Testing

• npm run build passes
• npm run test:jest passes (useDiffHydration.test.ts: 17/17; full jest suite: 4707/4724 passing — the 10 unrelated failing suites were SIGKILL/OOM from running the full suite under constrained sandbox memory, confirmed to pass individually and unrelated to any file touched here)
• npm run lint clean (0 errors; pre-existing exhaustive-deps warnings only, consistent with sibling hooks in this file)
• Added fake-timer unit tests: debounce (single fetch after 120ms), rapid re-selection (timer reset, single fetch), cancellation (cleanup before debounce elapses), and cache hit (no fetch, loading never toggles true)
• CI: pending

🤖 Generated by the harbor agent loop. Reviewed by a human before merge.

[gfargo-horizon-agent]

🔎 Agent review (sonnet) — LGTM

REVIEW: LGTM
RESOLVES: full

The diff correctly mirrors the already-merged useCommitDetailHydration debounce+cache pattern for the file-preview loader, preserving the active-flag guard, safe() wrapper, and hook-position constraints, with fake-timer tests covering debounce, cancellation, and cache-hit-skips-loading behavior. Scope is tightly limited to the two named files with no touches to app.ts or the sibling hook.